It is common practice to inspect work pieces subsequent to production on a coordinate positioning apparatus, such as a coordinate measuring machine (CMM), in order to check for correctness of predefined object parameters, like dimensions and shape of the object.
In a conventional 3-D coordinate measurement machine, a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in space of a measuring volume of the coordinate measuring machine and the object is measurable with a measurement sensor (probing unit) carried by the probe head.
In a simple form of the machine a suitable transducer mounted parallel to each axis is able to determine the position of the probe head relative to a base of the machine and, therefore, to determine the coordinates of a measurement point on the object being approached by the sensor, e.g. by the probe tip of the probing unit. For providing movability of the probe head a typical coordinate measuring machine may comprise a frame structure on which the probe head is arranged and driving means for moving frame components of the frame structure relative to each other.
For measuring surface variations, both measurement principles based on use of tactile sensors and of optical sensors are known.
In general, to provide a coordinate measuring machine with an improved measurement precision, its frame structure is therefore usually designed to have a high static stiffness. In order to achieve a stiff and rigid machine design, the frame structure or at least parts of it, is often made of stone, such as granite. Besides all the positive effects like thermal stability and good damping properties, the granite also makes the machine and the movable frame elements quite heavy. The high weight on the other side also requires high forces for a decent acceleration.
There are still several possible sources of error, if such technique is employed. Excited resonances or vibrations of machine parts when moving one frame component relative to another component are just two examples for dynamic errors. Moreover, errors emerging from vibrations coming from outside the machine are to be considered. Additionally, static errors like lack of straightness in movement and of orthogonality of the axes or lateral offset in the linear drive mechanisms may occur.
According to many approaches the mentioned errors are only analyzed statically, although they also comprise dynamic factors which are dependent on the movement of the axes, in particular dependent on the position, speed, acceleration and jerk when moving the axis. With the speed-dependent calibration, this fact is taken into account in a rather simple and inflexible way. While the static errors can be numerically reduced by the use of position calibration matrices, things get much more complex when trying to compensate the dynamic errors.
The calibration gets even more complex when taking into account the dynamic errors, such as mentioned vibrations or resonance or dynamic forces etc. which errors can not only influence the axis on which they are occurring, but which can also “crosstalk” to other axes and cause errors in other parts of the system. Furthermore, the underlying effects can also be dependent on environmental conditions such as temperature, humidity, air-pressure, etc. and in particular, they will also vary over the lifetime of the machine.
In that context, for example, it has to be considered that accelerations of one axis of the machine (which can move further perpendicular axes and the probe head), can cause linear and angular dynamic deflections of the whole frame of the coordinate measuring machine (e.g. of the probe head), which in turn cause measurement uncertainties and errors. These dynamic measurement errors may be reduced by taking measurements at low accelerations, e.g. by a consequently optimized trajectory of desired movement.
Exemplarily for error handling, EP 1 559 990 discloses a coordinate measuring system and method of correcting coordinates measured in a coordinate measuring machine, measuring geometrical errors while parts with various weights are mounted on the coordinate measuring machine. Compensation parameters are derived from measured results per a weight of a part and stored. A compensation parameter corresponding to a weight of a part to be measured is appropriately read out to correct measured coordinates of the part to be measured.
As a further example, EP 1 687 589 discloses a method of error compensation in a coordinate measuring machine with an articulating probe head having a surface detecting device. The surface detecting device is rotated about at least one axis of the articulating probe head during measurement. The method comprises the steps of: determining the stiffness of the whole or part of the apparatus, determining one or more factors which relate to the load applied by the articulating probe head at any particular instant and determining the measurement error at the surface sensing device caused by the load.
Another approach for error correction of work piece measurements with a coordinate measuring machine (CMM) is disclosed in GB 2 425 840. Thereby, position measurements are taken with a work piece sensing probe, in which means of measuring acceleration are provided. The measurements are corrected for both high frequency (unrepeatable) errors such as those due to vibration, and low frequency (repeatable) errors such as those due to centrifugal forces on the probe. The correction method comprises measuring the work piece, determining repeatable measurement errors from a predetermined error function, error map or error look-up table, measuring acceleration and calculating unrepeatable measurement errors, combining the first and second measurement errors to determine total errors and correcting the work piece measurements using the total errors. The predetermined error map is calculated using an artefact of known dimensions.
It is also known to use accelerometers fitted in the probe or on other moving parts of the measurement machine, e.g. the Z-column and/or in the base table, allowing a differential measurement and/or the evaluation of externally applied vibrations. In such an arrangement, the displacements and errors of the probe-position can be measured with double integration and, based on this information, it is possible to adjust the reading with the difference between the doubly integrated signal and the scales. For instance, such a quasi-static approach is disclosed by WO 02/04883.
However, according to above approaches, a displacement of the probe element when moving (accelerating) the probing unit is considered by computational way only.
Especially when using a scanning probe with the CMM and applying quite high moving speeds for measuring an object the dynamic effects which influence the probe element increase and introduce more errors to be considered (e.g. due to emerging touching forces when touching of the surfaces continuously). This leads to more complex computation resulting in errors not being fully compensatable by computation.
In general, the measuring tip of a scanning probe can be deflected in all three Cartesian directions (x,y,z), wherein sensors at the probing unit measure the deflections for each direction. In general, the deflections are allowed and measured by a “three flexure system” (one for each direction) sequentially connected to each other.
A main requirement for existing probes is to minimize the impact on the measuring part (object to be measured) and the machine—which means reducing the contact force. At the same time cross coupling between the axes has to be minimized and the linearity has to be maintained.
Low impact force, minimized cross coupling and linearity yields into weak (low resonances) and/or large probe structures.
Scanning performance (speed, acceleration) is therefore limited by the probing unit (or probe head) itself—as the resonances get excited, the linearity zone is left, dynamic forces due to accelerations lead to probe sensing element deflections and (more important) lift-off might happen.
In order to reduce such effects existing scanning probes damp contact issues with the help of viscous fluids. It is well known that the properties of those fluids depend on the environmental conditions and might change during lifetime and production. Thus, damping forces may vary over time and are not well known.
Furthermore those fluids can only exert passive forces correlated to the speed of deflection. Damping effects are therefore limited and designed for specific resonance frequencies which may occur with a respective measuring system. Lift-off effects at certain speeds and surfaces to be touched might still happen and cannot be prevented due to the passive characteristics of the existing probes (using viscous fluids).